1. Field of the Invention
The present invention relates to a method and apparatus for determining the extent of growth during the preparation of a crystalline material, such as sapphire or silicon.
2. Description of the Related Art
Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as alumina or silicon, in a crucible to produce an ingot. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce an ingot, such as a sapphire ingot, by the HEM method, solid feedstock, such as alumina, is provided in a crucible containing a monocrystalline seed (which comprises the same material as the feedstock but with a single crystal orientation throughout) placed into the hot zone of a solidification furnace. A heat exchanger, such as a helium-cooled heat exchanger, is positioned in thermal communication with the crucible bottom and with the monocrystalline seed. The feedstock is then heated to form a liquid feedstock melt, without substantially melting the monocrystalline seed, and heat is then removed from the melted feedstock by applying a temperature gradient in the hot zone in order to directionally solidify the melt from the unmelted seed. By controlling how the melt solidifies, a crystalline material having a crystal orientation corresponding to that of the monocrystalline seed, and having greater purity than the starting feedstock material, can be achieved.
In such crystallization methods, it is important to monitor the extent of solidification in order to produce a high quality final product. Variations in the rate of growth can result in significant variations throughout the growing crystalline material, and such variability can reduce the amount of useful material in the final ingot. However, it is often a challenge to efficiently and accurately identify how much of the melted feedstock has solidified throughout the growth phase. Typically, the extent of crystal growth is determined manually by inserting a dip rod into the melt at various time intervals and measuring the penetration depth. However, the dip rod method has significant disadvantages. For example, dip rods can be easily broken in the partially solidified material or, at the very least, can introduce impurities into the final product. Also, this manual method is time and labor intensive, requiring the presence of skilled operators, thereby increasing the overall cost of ingot production. Finally, dip rod data cannot efficiently be used for process control. Thus, if a difference is found between the measured growth rate and the expected or targeted growth rate, changes to the process cannot be made instantaneously.
Thus, there is a need in the industry for methods and devices that can be used to efficiently and effectively monitor the directional solidification of a feedstock melt in a crystal growth apparatus, in order to ensure a consist growth process.